src: update NODE_MODULE_VERSION to 57

Node.js 8.0.0 is API/ABI forward-compatible with V8 6.0.
The current module version is 55 and corresponds to V8 5.8. This
increments the number by two to prevent incompatibility with modules
compiled from Node master.

Author: targos

src/node_version.h

@@ -73,6 +73,6 @@
  * an API is broken in the C++ side, including in v8 or
  * other dependencies.
  */
-#define NODE_MODULE_VERSION 55 /* Node.js v8.0.0 */
+#define NODE_MODULE_VERSION 57 /* Node.js v8.0.0 */
 
 #endif  // SRC_NODE_VERSION_H_